Process for making pulp

ABSTRACT

A process for making pulp includes providing plant fiber raw material. The plant fiber raw material can be wood fiber or gramineae plant fiber. The plat fiber raw material is cooked to make a pulp. A TEMPO catalytic oxidation system is added to the pulp to oxidize the pulp fiber. The oxidized pulp fiber is then bleached.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority to People's Republic of China Patent Application No. 201310129660.5, filed Apr. 15, 2013, which is hereby incorporated by reference herein in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to a pulp-making process.

2. Description of Related Art

Cellulose is an organic compound with the formula (C₆H₁₀O₅)_(n). Cellulose is a polysaccharide consisting of a linear chain of several hundred to over ten thousand P-1,4 linked D-glucose units, wherein each glucose unit has a primary hydroxyl group at carbon 6, a secondary hydroxyl group at carbon 2, and a secondary hydroxyl group at carbon 3. The hydroxyl groups of the cellulose can be selectively oxidized to carboxyl groups (—COOH) or aldehyde groups (—CHO). In the field of papermaking, cellulose with aldehyde groups improves the wet strength of the paper, while cellulose with carboxyl groups promotes swelling of the fibers, which is beneficial for the beating of the pulp, and improves the strength of the paper.

The traditional pulp-making process includes at least the following steps: providing plant fiber raw material; cooking the plant fiber raw material to be a pulp; and bleaching the pulp. However, the content of the carboxyl groups generated during the traditional pulp-making process is less than 0.1 millimole per gram (mmol/g) of pulp fiber by dry weight. Thus, the advantage brought by the carboxyl groups and the aldehyde groups is limited.

Therefore, there is room for improvement within the art.

DETAILED DESCRIPTION

A pulp-making process includes the following steps.

Step 1: providing plant fiber raw material. The plant fiber raw material can be selected from wood fiber, gramineae plant fiber, or any combination thereof

The plant fiber raw material is cleaned, debarked, unknotted, and cut into pieces in advance for the following cooking step.

Step 2: cooking the plant fiber raw material to make a pulp.

The cooking step in this disclosure uses methods known in the art, such as the Kraft pulping method for the wood fiber, or the soda-anthraquinone method for the gramineae plant fiber, for example. For wood fibers, most lignins contained in the wood fibers are removed by the cooking process.

The pulp is washed and filtered, thereby removing impurities from the pulp. The impurities may be fiber bundles, uncooked fiber pieces, mud, or iron filings, for example.

Step 3: adding a TEMPO catalytic oxidation system into the pulp to oxidize the pulp fiber. The TEMPO catalytic oxidation system is added into the pulp after the pulp is washed and filtered. The oxidation process removes any residual lignins in the wood pulp. The hydroxyl groups at carbon 6 of the glucose chains of the fibers are substantially oxidized to carboxyl groups (—COOH), while only a small amount of hydroxyl groups are oxidized to aldehyde groups (—CHO). The final pulp is used for making paper. Alternatively, the final pulp can be used for making cellulose ether, cellulose ester, or any derivatives thereof

The TEMPO catalytic oxidation system contains a catalyst, an assistant catalyst, and an oxidant. The catalyst is TEMPO ((2,2,6,6-Tetramethylpiperidin-1-yl)oxyl) or a derivative of TEMPO. The derivative of TEMPO may be 4-(acetylamino)-TEMPO. The weight of catalyst is about 0.01% to about 2% of the dry weight of the pulp fiber. The assistant catalyst consists of iodides, bromides, borates, or a mixture thereof In one embodiment, the assistant catalyst is sodium bromide. The weight of assistant catalyst is about 0.1% to about 10% of the dry weight of the pulp fiber. The oxidant can be a dioxide, peroxide, chlorite, or hypochlorite. In one embodiment, the oxidant is sodium hypochlorite. The content of the oxidant is in the range from 0.2 to 1.5 mmol per gram of pulp fiber by dry weight.

The pH value of the pulp is kept between about 9.5 and about 10.5, and the temperature of the pulp is kept to be about 0° C. to about 110° C. during the oxidation process. In one embodiment, the temperature of the pulp is kept to be about 20° C. to about 70° C. during the oxidation process. The oxidation step lasts for about 10 to about 180 min.

Step 4: bleaching the pulp. The bleaching step in this disclosure uses methods known in the art, such as the oxidation bleaching method or the reduction bleaching method. In this embodiment, the bleaching step uses the elemental chlorine free (ECF) bleaching method in which chlorine dioxide is used as a bleaching agent. In other embodiments, the step of bleaching the pulp may be carried out after the cooking step and before the step of adding the oxidant system.

In other embodiments, the oxidation step can be carried out by an oxidant selected from the periodates, nitrites, sodium chlorate, or sodium hypochlorite if no catalysts are available. The advantage to using the TEMPO catalytic oxidation system is that it uses catalysts to speed up the oxidation of the hydroxyl groups.

The oxidant or the TEMPO catalytic oxidation system is added into the pulp after the cooking step and before the bleaching step because the oxidation process also whitens the pulp a little. Therefore, the dosage of the bleaching agent in the bleaching step can be effectively reduced, and the residual lignins are effectively removed.

The pulp made by the above-described pulp-making process contains carboxyl groups and aldehyde groups. The content of the carboxyl groups in the pulp is in the range from about 0.1 to about 1.5 mmol per gram of pulp fiber by dry weight. The content of the aldehyde groups in the pulp is in the range from about 0.05 to about 0.7 mmol per gram of pulp fiber by dry weight.

The pulp-making process provided in the disclosure provides pulp having a high content of carboxyl groups and aldehyde groups. The aldehyde groups improve the wet strength of the paper; and the carboxyl groups reduce the energy required to beat the pulp and improve the strength of the paper. Furthermore, the pulp having a high content of carboxyl groups and aldehyde groups can be used for making cellulose ethers, cellulose esters, or any derivatives thereof.

EXAMPLE 1

In this embodiment, softwood pieces were cooked by the Kraft pulping method to make a pulp, wherein the content of alkali used in the cooking step was about 20% of the dry weight of the pulp fiber, sulfidity was 24%, and the liquor-to-wood ratio was 1:4. The softwood pieces were cooked at a temperature of 170 ° C. for 180 min. The TEMPO catalytic oxidation system was added into the pulp, wherein the content of the TEMPO was about 0.5% of the dry weight of the pulp fiber, the content of the sodium bromide was about 5% of the dry weight of the pulp fiber, the content of the sodium hypochlorite was 1.0 mmol per gram of pulp fiber by dry weight, the pH value of the pulp was kept between about 10 and about 10.5, and the temperature of the pulp was kept to be 30° C.

The oxidized pulp was bleached by OD₀(EO)D₁D₂ five bleaching steps in order. For the O bleaching step, consistency of the pulp was 10%, content of sodium hydroxide was 3% of the dry weight of the pulp fiber, oxygen pressure was 0.6 MPa, temperature was 90° C., and time lasted for 60 min. For the D₀ bleaching step, consistency of the pulp was 10%, content of chlorine dioxide was 1.3% of the dry weight of the pulp fiber, temperature was 50° C., initial pH value was 2.5, and time lasted for 45 min. For the EO bleaching step, consistency of the pulp was 10%, content of sodium hydroxide was 3% of the dry weight of the pulp fiber, oxygen pressure was 0.3 MPa, temperature was 90° C., initial pH value was 11, and time lasted for 40 min. For the D₁ bleaching step, consistency of the pulp was 6%, content of chlorine dioxide was 0.8% of the dry weight of the pulp fiber, temperature was 70° C., final pH value was 3.5 to 4.0, and time lasted for 180 min. For the D2 bleaching step, consistency of the pulp was 10%, content of chlorine dioxide was 0.4% of the dry weight of the pulp fiber, temperature was 70° C., final pH value was 4.0 to 4.5, and time lasted for 180 min.

The content of the carboxyl groups in the pulp was 0.2 mmol per gram of pulp fiber by dry weight; the content of the aldehyde groups in the pulp was 0.11 mmol per gram of pulp fiber by dry weight.

EXAMPLE 2

In this embodiment, hardwood pieces were cooked by the Kraft pulping method to make a pulp, wherein the content of alkali used in the cooking step was about 16% of the dry weight of the pulp fiber, sulfidity was 20%, and the liquor-to-wood ratio was 1:4. The temperature of the hardwood was elevated to 170° C. from room temperature for 90 min, and then the hardwood was cooked at 170° C. for 150 min. The TEMPO catalytic oxidation system was added into the pulp, wherein the content of the TEMPO was about 0.5% of the dry weight of the pulp fiber, the content of the sodium bromide was about 5% of the dry weight of the pulp fiber, the content of the sodium hypochlorite was 6 mmol per gram of pulp fiber by dry weight, the pH value of the pulp was kept between about 10 and about 10.5, and the temperature of the pulp was kept to be 30° C.

The oxidized pulp was bleached by OD₀(EO)D₁ four bleaching steps in order. For the O bleaching step, consistency of the pulp was 10%, content of sodium hydroxide was 3% of the dry weight of the pulp fiber, oxygen pressure was 0.4 MPa, temperature was 90° C., initial pH value was 10 to 11, and time lasted for 60 min. For the D₀ bleaching step, consistency of the pulp was 10%, content of chlorine dioxide was 1.0% of the dry weight of the pulp fiber, temperature was 50° C., initial pH value was 2.5, and time lasted for 30 min. For the EO bleaching step, consistency of the pulp was 10%, content of sodium hydroxide was 3% of the dry weight of the pulp fiber, oxygen pressure was 0.15 MPa, temperature was 70° C., initial pH value was 11, and time lasted for 60 min. For the D₁ bleaching step, consistency of the pulp was 10%, content of chlorine dioxide was 1.2% of the dry weight of the pulp fiber, temperature was 70° C., final pH value was 3.0 to 4.0, and time lasted for 180 min.

The content of the carboxyl groups in the pulp was 1.02 mmol per gram of pulp fiber by dry weight; the content of the aldehyde groups in the pulp was 0.58 mmol per gram of pulp fiber by dry weight.

EXAMPLE 3

In this embodiment, hardwood pieces were cooked by the Kraft pulping method to make a pulp, wherein the content of alkali used in the cooking step was about 16% of the dry weight of the pulp fiber, sulfidity was 20%, and the liquor-to-wood ratio was 1:4. The temperature of the hardwood was elevated to 170° C. from room temperature for 90 min, and then the hardwood was cooked at 170° C. for 150 min. The TEMPO catalytic oxidation system was added into the pulp, wherein the content of the TEMPO was about 0.5% of the dry weight of the pulp fiber, the content of the sodium bromide was about 5% of the dry weight of the pulp fiber, the content of the sodium hypochlorite was 10 mmol per gram of pulp fiber by dry weight, the pH value of the pulp was kept to be between about 10 and about 10.5, and the temperature of the pulp was kept to be 30° C.

The oxidized pulp was bleached by OD₀(EO)D₁ four bleaching steps in order. For the O bleaching step, consistency of the pulp was 10%, content of sodium hydroxide was 3% of the dry weight of the pulp fiber, oxygen pressure was 0.4 MPa, temperature was 90° C., initial pH value was 10 to 11, and time lasted for 60 min. For the D₀ bleaching step, consistency of the pulp was 10%, content of chlorine dioxide was 1.0% of the dry weight of the pulp fiber, temperature was 50° C., initial pH value was 2.5, and time lasted for 30 min. For the EO bleaching step, consistency of the pulp was 10%, content of sodium hydroxide was 3% of the dry weight of the pulp fiber, oxygen pressure was 0.15 MPa, temperature was 70° C., initial pH value was 11, and time lasted for 60 min. For the D₁ bleaching step, consistency of the pulp was 10%, content of chlorine dioxide was 1.2% of the dry weight of the pulp fiber, temperature was 70° C., final pH value was 3.0 to 4.0, and time lasted for 180 min.

The content of the carboxyl groups in the pulp was 1.51 mmol per gram of pulp fiber by dry weight; the content of the aldehyde groups in the pulp was 0.76 mmol per gram of pulp fiber by dry weight.

EXAMPLE 4

In this embodiment, ryegrass was cooked by the soda-anthraquinone method to make a pulp, wherein the content of sodium hydroxide used in the cooking step was about 16% of the dry weight of the pulp fiber, the content of anthraquinone used in the cooking step was about 0.05% of the dry weight of the pulp fiber, and the liquor-to-grass ratio was 1:4. The temperature of the ryegrass was elevated to 160° C. from room temperature for 120 min, and then the ryegrass was cooked at 160° C. for 50 min. The TEMPO catalytic oxidation system was added into the pulp, wherein the content of the TEMPO was about 0.5% of the dry weight of the pulp fiber, the content of the sodium bromide was about 5% of the dry weight of the pulp fiber, the content of the sodium hypochlorite was 6 mmol per gram of pulp fiber by dry weight, the pH value of the pulp was kept to be between about 10 and about 10.5, and the temperature of the pulp was kept to be 30° C.

The oxidized pulp was bleached by D₀(EOP)D₁ three bleaching steps in order. For the D₀ bleaching step, consistency of the pulp was 10%, content of chlorine dioxide was 1.0% of the dry weight of the pulp fiber, temperature was 60° C., final pH value was 4.0 to 4.5, and time lasted for 60 min. For the EOP bleaching step, consistency of the pulp was 10%, content of sodium hydroxide was 3% of the dry weight of the pulp fiber, oxygen pressure was 0.3 MPa, temperature was 70° C., initial pH value was 11, and time lasted for 60 min. For the D₁ bleaching step, consistency of the pulp was 10%, content of chlorine dioxide was 1.2% of the dry weight of the pulp fiber, temperature was 70° C., final pH value was between about 3.5 and about 4.0, and time lasted for 120 min.

The content of the carboxyl groups in the pulp was 0.97 mmol per gram of pulp fiber by dry weight; the content of the aldehyde groups in the pulp was 0.54 mmol per gram of pulp fiber by dry weight.

EXAMPLE 5

In this embodiment, ryegrass was cooked by the soda-anthraquinone method to make a pulp, wherein the content of sodium hydroxide used in the cooking step was about 16% of the dry weight of the pulp fiber, the content of anthraquinone used in the cooking step was about 0.05% of the dry weight of the pulp fiber, and the liquor-to-grass ratio was 1:4. The temperature of the ryegrass was elevated to 160° C. from room temperature for 120 min, and then the ryegrass was cooked at 160° C. for 50 min.

The pulp was bleached by D₀(EOP)D₁ three bleaching steps in order. For the D₀ bleaching step, consistency of the pulp was 10%, content of chlorine dioxide was 1.0% of the dry weight of the pulp fiber, temperature was 60° C., final pH value was 4.0 to 4.5, and time lasted for 60 min. For the EOP bleaching step, consistency of the pulp was 10%, content of sodium hydroxide was 3% of the dry weight of the pulp fiber, oxygen pressure was 0.3 MPa, temperature was 70° C., initial pH value was 11, and time lasted for 60 min. For the D₁ bleaching step, consistency of the pulp was 10%, content of chlorine dioxide was 1.2% of the dry weight of the pulp fiber, temperature was 70° C., final pH value was between about 3.5 and about 4.0, and time lasted for 120 min.

The TEMPO catalytic oxidation system was added into the bleached pulp, wherein the content of the TEMPO was about 0.5% of the dry weight of the pulp fiber, the content of the sodium bromide was about 5% of the dry weight of the pulp fiber, the content of the sodium hypochlorite was 6 mmol per gram of pulp fiber by dry weight, the pH value of the pulp was kept to be between about 10 and about 10.5, and the temperature of the pulp was kept to be 30° C.

The content of the carboxyl groups in the pulp was 1.00 mmol per gram of pulp fiber by dry weight; the content of the aldehyde groups in the pulp was 0.57 mmol per gram of pulp fiber by dry weight.

COMPARATIVE EXAMPLE 1

In this embodiment, hardwood pieces were cooked by the Kraft pulping method to make a pulp, wherein the content of alkali used in the cooking step was about 16% of the dry weight of the pulp fiber, sulfidity was 20%, and the liquor-to-wood ratio was 1:4. The temperature of the hardwood was elevated to 170° C. from room temperature for 90 min, and then the hardwood was cooked at 170° C. for 150 min.

The pulp was bleached by OD_(o)(EO)D₁ four bleaching steps in order. For the O bleaching step, consistency of the pulp was 10%, content of sodium hydroxide was 3% of the dry weight of the pulp fiber, oxygen pressure was 0.4 MPa, temperature was 90° C., initial pH value was 10 to 11, and time lasted for 60 min. For the D₀ bleaching step, consistency of the pulp was 10%, content of chlorine dioxide was 1.0% of the dry weight of the pulp fiber, temperature was 50° C., initial pH value was 2.5, and time lasted for 30 min. For the EO bleaching step, consistency of the pulp was 10%, content of sodium hydroxide was 3% of the dry weight of the pulp fiber, oxygen pressure was 0.15 MPa, temperature was 70° C., initial pH value was 11, and time lasted for 60 min. For the D₁ bleaching step, consistency of the pulp was 10%, content of chlorine dioxide was 1.2% of the dry weight of the pulp fiber, temperature was 70° C., final pH value was 3.0 to 4.0, and time lasted for 180 min.

The content of the carboxyl groups in the pulp was 0.06 mmol per gram of pulp fiber by dry weight; the pulp had no aldehyde group.

It is believed that the exemplary embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the disclosure or sacrificing all of its advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the disclosure. 

1. A process for making pulp, comprising: providing plant fiber raw material; cooking the plant fiber raw material to make a pulp; adding a TEMPO catalytic oxidation system into the pulp to oxidize the pulp fiber; and bleaching the oxidized pulp.
 2. The process as claimed in claim 1, wherein the plant fiber raw material is selected from the group consisting of wood fiber, gramineae plant fiber, and any combination thereof
 3. The process as claimed in claim 1, wherein the process further includes a step of washing the pulp and a step of filtering the pulp after the step of cooking and before the step of adding a TEMPO catalytic oxidation system into the pulp.
 4. The process as claimed in claim 1, wherein the process further includes a step of bleaching the pulp after the step of cooking the plant fiber raw material, the TEMPO catalytic oxidation system is added into the pulp after the step of bleaching the pulp.
 5. The process as claimed in claim 1, wherein the TEMPO catalytic oxidation system contains a catalyst, an assistant catalyst, and an oxidant; the catalyst is TEMPO ((2,2,6,6-Tetramethylpiperidin-1-yl)oxyl) or a derivative thereof; the weight of catalyst is about 0.01% to about 2% of the dry weight of the pulp fiber; the assistant catalyst consists of iodides, bromides, borates, or a mixture thereof; the weight of assistant catalyst is about 0.1% to about 10% of the dry weight of the pulp fiber; the oxidant is chlorine dioxide, hydrogen peroxide, chlorite, or hypochlorite; the content of the oxidant is in the range from about 0.2 mmol to about 1.5 mmol per gram of pulp fiber by dry weight.
 6. The process as claimed in claim 5, wherein the derivative of TEMPO is 4-(acetylamino)-TEMPO.
 7. The process as claimed in claim 1, wherein the pH value of the pulp is kept to be between about 9.5 and about 10.5, and the temperature of the pulp is kept to be about 0° C. to about 110° C. during the oxidation process.
 8. The process as claimed in claim 1, wherein the pH value of the pulp is kept to be between about 9.5 and about 10.5, and the temperature of the pulp is kept to be about 20° C. to about 70° C. during the oxidation process.
 9. The process as claimed in claim 1, wherein the oxidation step is performed for a time period from about 10 to about 180 min.
 10. The process as claimed in claim 1, wherein the pulp contains carboxyl groups and aldehyde groups; the content of the carboxyl groups in the pulp is in the range from about 0.1 mmol to about 1.5 mmol per gram of pulp fiber by dry weight; the content of the aldehyde groups in the pulp is in the range from about 0.05 mmol to about 0.7 mmol of per gram pulp fiber by dry weight.
 11. A process for making pulp, comprising: providing plant fiber raw material; cooking the plant fiber raw material to make a pulp; adding an oxidant into the pulp to oxidize the pulp fiber, the oxidant being selected from the group consisting of periodate, nitrite, sodium chlorate, and sodium hypochlorite; and bleaching the oxidized pulp.
 12. The process as claimed in claim 11, wherein the plant fiber raw material is selected from the group consisting of wood fiber, gramineae plant fiber, and any combination thereof.
 13. The process as claimed in claim 11, wherein the process further includes a step of washing the pulp and a step of filtering the pulp after the step of cooking and before the step of adding an oxidant into the pulp.
 14. The process as claimed in claim 11, wherein the process further includes a step of bleaching the pulp after the step of cooking the plant fiber raw material, the oxidant is added into the pulp after the step of bleaching the pulp.
 15. The process as claimed in claim 11, wherein the pulp contains carboxyl groups and aldehyde groups; the content of the carboxyl groups in the pulp is in the range from about 0.1 mmol to about 1.5 mmol per gram of pulp fiber by dry weight; the content of the aldehyde groups in the pulp is in the range from about 0.05 mmol to about 0.7 mmol per gram of pulp fiber by dry weight. 